Cavity-backed slot antenna with an active artificial magnetic conductor

CROSS REFERENCE TO RELATED APPLICATIONS

0001This application claims the benefit of U.S. provisional patent application No. 61/655,670 filed Jun. 5, 2012, the disclosure of which is hereby incorporated by reference.

0002This application is also related to U.S. patent application Ser. No. 13/441,730 filed Apr. 6, 2012 and entitled “Differential Negative Impedance Converters and Inverters with Tunable Conversion Ratios”, the disclosure of which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

0003None.

TECHNICAL FIELD

0004This invention relates to cavity backed antennas.

BACKGROUND

0005Cavity-backed slot antennas (CBSA) have been extensively investigated for applications to airborne and satellite communications because they satisfy the requirements of flush mounting, low cost and light weight. Their optimum size scales with the wavelength of the desired radiation frequency which the antenna transmits and/or receives. In order to get the antenna to radiate efficiently, the cavity height is usually designed to be one- or three-quarter wavelengths at the resonator frequency in order not to destroy impedance matching. At low frequencies, such as the VHF and UHF bands, where the radiation wavelength is 1 m or longer, the CBSA can be very large and hard to mount on aircraft. Embodiments of the principles of the present invention described below comprise a reduced-size CBSA that radiates efficiently at low frequencies over a large bandwidth with a tunable operation band.

0006The prior art teaches that the CBSA cavity height can be reduced through dielectric loading but then the bandwidth and efficiency will also be reduced.

0007Itoh and Yang (U.S. Pat. No. 6,518,930) have disclosed a CB SA loaded with a passive Artificial Magnetic Conductor (AMC) structure. The AMC transforms the cavity ground plane into an electrically open surface, and allows the CBSA to operate at lower frequencies without an excessively deep cavity. However, the measured bandwidth of the antenna is very narrow because they use the passive AMC structure to load the CBSA.

BRIEF DESCRIPTION OF THE INVENTION

0008The invention is a low-profile, cavity-backed slot antenna loaded with an active artificial magnetic conductor (AAMC). The invention uses an AMC that is loaded with reactive members and preferably with non-Foster ICs (NFC) that provide a negative inductance. Some embodiments according to the principles of the present invention demonstrate that NFCs added to the AAMC grid increases the bandwidth by more than a factor of ten over a passive AMC.

0009In one embodiment according to the principles of the present invention, a very high frequency (VHF) CBSA with the AAMC demonstrated that it enables efficient radiation over a significantly wide bandwidth, unreported in the prior art. Since the NFC is tunable with an applied voltage, the AAMC-CBSA is tunable also. One embodiment according to the principles of the present invention is tunable from 260 MHz to 350 MHz.

0010The prior art embodiments show an AMC-CBSA and a very wide cavity with respect to the cavity length, i.e it has a large width to length aspect ratio and requires an AMC that is several unit cells across. Embodiments according to the principles of the present invention are narrow, less than 1/10 wavelength, and only require a single unit cell across the width.

0011In one aspect the present invention provides a cavity-backed slot antenna whose cavity has an artificial magnetic conductor (AMC) disposed therein, the AMC being formed by an array of metal patches displaced by a distance above a bottom of said cavity, the metal patches have edges confronting sidewalls of the cavity, said edges being electrically connected to said sidewalls, the AMC being loaded with active reactive elements.

0012In another aspect the present invention provides a cavity-backed slot antenna whose cavity has an artificial magnetic conductor (AMC) disposed therein, the AMC comprising an array of metal patches displaced by a set distance above a bottom of said cavity, the metal patches being arrayed in two columns running along a length of the cavity, and with a gap between the columns, the metal patches having edges confronting sidewalls of the cavity, said edges being electrically connected to said sidewalls, each gap between neighboring patches being bridged by reactive elements.

0013In yet another aspect the present invention provides a cavity-backed slot antenna whose cavity has an artificial magnetic conductor (AMC) disposed therein, the AMC comprising an array of metal patches displaced by a set distance above a bottom of said cavity, the metal patches being arrayed in a single column running along a length of the cavity, and with a gap between the column and sidewalls of the cavity, the metal patches having edges confronting sidewalls of the cavity, said edges being electrically coupled to said sidewalls via reactive elements. <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0014">In still yet another aspect the present invention provides a method of lowering a resonant frequency of a cavity backed slot antenna comprising the steps of: disposing a plurality of electrically conductive patches in a cavity of said cavity backed slot antenna adjacent a slot of said cavity backed slot antenna; and coupling capacitive elements (a) between opposing or neighboring ones of said electrically conductive patches and/or (b) between said plurality of electrically conductive patches and an electrically conductive wall defining at least two edges of said cavity.</li></ul></li></ul>

0015In yet another aspect the present invention provides a method of increasing the bandwidth around a resonant frequency of a cavity backed slot antenna comprising the steps of: disposing an array of electrically conductive patches in a cavity of said cavity backed slot antenna adjacent a slot of said cavity backed slot antenna, the array of electrically conductive patches forming an artificial magnetic conductor; and coupling capacitive elements (a) between opposing ones of said electrically conductive patches and/or (b) between said plurality of electrically conductive patches and an electrically conductive wall defining at least two edges of said cavity, said capacitive elements each having a negative capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

0016<figref idref="DRAWINGS">FIGS. 1A, 1B and 1C</figref> depict atop view and two sectional views, respectively, of an embodiment of a cavity-backed slot antenna (CBSA) loaded with an active artificial magnetic conductor (AAMC);

0017<figref idref="DRAWINGS">FIG. 2</figref> is a perspective view of the cavity showing one embodiment of an AAMC therein;

0018<figref idref="DRAWINGS">FIG. 3</figref> is a perspective view of the AAMC that is inserted into the antenna cavity of <figref idref="DRAWINGS">FIG. 2</figref>, for example;

0019<figref idref="DRAWINGS">FIG. 4</figref> is a top view of the AAMC inside the antenna cavity;

0020<figref idref="DRAWINGS">FIG. 5</figref> is a side elevational view through an embodiment of the slot showing two patches of the array of patches with a fixed reactive element coupling them in the cavity behind the slot and a coaxial feed across the slot.

0021<figref idref="DRAWINGS">FIG. 6</figref> is a side elevational view through an embodiment of the slot showing two patches of the array of patches with a variable reactive element coupling them in the cavity behind the slot.

0022<figref idref="DRAWINGS">FIG. 7</figref> is a side elevational view through an embodiment of the slot showing two patches of the array of patches with a variable non-Foster circuit element coupling them in the cavity behind the slot.

0023<figref idref="DRAWINGS">FIG. 8</figref> is an alternative design, similar to that of <figref idref="DRAWINGS">FIG. 4</figref>, but in this case the reactive elements couple the sides of a single row of patches to the walls of the cavity.

0024<figref idref="DRAWINGS">FIGS. 9A-9E</figref> are photographs of a AAMC-CBSA test article which was made and tested.

0025<figref idref="DRAWINGS">FIGS. 10A and 10B</figref> are graphs depicting the test results for the AAMC-CBSA test article of <figref idref="DRAWINGS">FIGS. 9A-9E</figref>.

0026<figref idref="DRAWINGS">FIG. 11A</figref> is a schematic diagram of a preferred embodiment of a Non-Foster Circuit.

0027<figref idref="DRAWINGS">FIGS. 11B and 11C</figref> depict measured circuit values for the NFC of <figref idref="DRAWINGS">FIG. 11A</figref>.

DETAILED DESCRIPTION

0028One embodiment according to the principles of the present invention, comprises a cavity-backed slot antenna (CBSA) loaded with an active artificial magnetic conductor (AAMC). The AAMC is an artificial magnetic conductor (AMC) loaded with negative inductance non-Foster circuits (NFCs).

0029Referring to <figref idref="DRAWINGS">FIGS. 1A, 1B and 1C</figref>, <figref idref="DRAWINGS">FIG. 1A</figref> depicts a top view of the AAMC-CBSA while <figref idref="DRAWINGS">FIGS. 1B and 1C</figref> depict side sectional views taken along lines A-A′ and B-B′ shown in <figref idref="DRAWINGS">FIG. 1A</figref>. The AAMC-CBSA is formed by a slot <b>102</b> in a metal plate <b>101</b> which typically acts as a ground plane for the antenna. The slot <b>102</b> is open to a cavity <b>100</b> below it (it is open in an electrical sense in that the slot <b>102</b> and/or the cavity <b>100</b> below it may be filled or partially filled with an electrically transparent or translucent material such as a dielectric material).

0030An AMC <b>103</b> is disposed in the cavity <b>100</b> and preferably fills the cavity by extending towards all four sides of the cavity <b>100</b>, the sides of the cavity <b>100</b> comprising cavity walls <b>105</b> which are represented by the dashed lines associated with numeral <b>105</b> in <figref idref="DRAWINGS">FIG. 1A</figref> and solid lines <b>105</b> in <figref idref="DRAWINGS">FIGS. 1B and 1C</figref>. The AMC <b>103</b> may comprise a reactive metallic grid of patches (see, e.g., patches <b>204</b> in <figref idref="DRAWINGS">FIG. 2</figref>), for example, and preferably has a dielectric substrate <b>104</b> disposed preferably below, but usually on at least one side of the grid of patches, preferably to provide a physical support for the patches. The patches may also or alternatively be supported directly or indirectly by the walls <b>105</b> of the cavity <b>100</b>. When loaded with active circuits, such as NFCs, then the AMC <b>103</b> can be called a AAMC. The AMC <b>103</b> is preferably disposed a fixed distance <b>115</b> above the floor <b>111</b> of the cavity <b>100</b>.

0031The length <b>110</b> of the cavity <b>100</b> is approximately one wavelength long for the desired radiation frequency which the antenna transmits and/or receives, while the width <b>108</b> of the cavity <b>100</b> is less in this embodiment (but lengths <b>108</b>, <b>110</b> of the cavity <b>100</b> could be the same size or nearly the same size in other embodiments). The slot <b>108</b> can be as long as the cavity <b>100</b> or shorter than the cavity <b>100</b> (as is the case in <figref idref="DRAWINGS">FIGS. 1A and 1B</figref>), but preferably it should not be longer than the cavity <b>100</b>. The width of slot <b>102</b> is usually very narrow compared to the cavity's width <b>108</b>. The CBSA can be excited in a variety of ways well known in the art. One embodiment according to the principles of the present invention uses a coaxial cable <b>106</b> whose ground shield is coupled (for example, by soldering) to one side of the slot <b>102</b> while the coax cable's center conductor <b>107</b> is connected (for example, by soldering) to the other side of the slot <b>102</b>.

0032The width <b>108</b> and depth <b>109</b> of the cavity can be any convenient size. However, in order to make a low-profile antenna, it is preferable if the width <b>108</b> and the depth <b>109</b> of the of the cavity <b>100</b> are less than 1/10 a wavelength for the desired radiation frequency.

0033Referring to <figref idref="DRAWINGS">FIG. 2</figref>, an AMC <b>203</b> is shown in this cutaway three dimensional view sitting in the cavity <b>100</b>, but this embodiment of the AMC <b>203</b> has a different aspect ratio (length to width) compared to the AMC <b>103</b> of <figref idref="DRAWINGS">FIGS. 1A-1C</figref>. Also the slot <b>102</b> depicted in <figref idref="DRAWINGS">FIGS. 1A-1C</figref> is only shown as two dashed lines <b>102</b> in this figure to better show the details of the AMC <b>203</b> below the slot <b>102</b> in this embodiment. This embodiment includes a slot <b>102</b> which has a width smaller than the width (<b>108</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) of the depicted AMC <b>203</b> that sits inside the cavity <b>100</b> a set distance (<b>115</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) above the floor <b>111</b> of the cavity <b>100</b>. In this embodiment the AMC <b>203</b> is formed by a plurality of metallic patches <b>201</b> disposed on a dielectric substrate <b>204</b> (dielectric substrate <b>204</b> may serve same the function as the dielectric substrate <b>104</b> mentioned above). The substrate <b>204</b> can be circuit board material or it can fill all or a portion of the cavity <b>100</b>. The patches <b>201</b> in the AMC <b>203</b> of this embodiment are aligned in two columns along the length of the cavity <b>100</b>, with a gap g between adjacent patches <b>201</b> in each column and in each row thereof. The two columns span the width (<b>108</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) of the cavity <b>100</b>. The size of the patches and the gap g between them, and the distance (<b>115</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) between the array of patches <b>201</b> and the cavity floor (<b>111</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) all influence the resonant frequency and bandwidth of the antenna, as is well known to those familiar with AMC technology. The patch <b>201</b> shape also affects the antenna performance. In the accompanying figures, and in a test article discussed below, embodiments of the principles of the present invention use rectangular patches but other geometric shapes can be used if desired for patches <b>201</b>. The sides of the patches <b>201</b> in the embodiment of <figref idref="DRAWINGS">FIG. 2</figref> are electrically connected to the walls of the CBSA cavity <b>100</b>.

0034<figref idref="DRAWINGS">FIG. 3</figref> shows the AMC of <figref idref="DRAWINGS">FIG. 2</figref> but with plate <b>101</b> and cavity <b>100</b> omitted for ease of illustration. Pairs of patches <b>201</b> are connected by a reactive element <b>202</b> as shown in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>. <figref idref="DRAWINGS">FIG. 4</figref> shows a top view of the embodiment in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>. Alternative embodiments may use various reactive elements <b>202</b>. For example, and not to imply a limitation, each reactive element <b>202</b> can be embodied as capacitive element such as a fixed capacitor <b>501</b> as shown in <figref idref="DRAWINGS">FIG. 5</figref>, or as a variable varactor <b>601</b> as shown in <figref idref="DRAWINGS">FIG. 6</figref>, or as an active non-Foster circuit <b>701</b> as shown in <figref idref="DRAWINGS">FIG. 7</figref>. In the case where a fixed capacitor <b>501</b> is used to load the grid, the CBSA's resonant frequency can be lowered to a much lower frequency than a CBSA with an unloaded cavity, but the bandwidth decreases as the frequency is lowered. The higher the capacitance value used, the lower the frequency. The variable varactor <b>601</b> in <figref idref="DRAWINGS">FIG. 6</figref> may be formed of two diodes biased by an adjustable control voltage <b>602</b> applied to conductor <b>604</b>.

0035<figref idref="DRAWINGS">FIG. 5</figref> is a side elevation view through the CBSA showing one of the the capacitors <b>501</b> and also the shows the CBSA being excited by cable <b>106</b>. In this embodiment member <b>101</b> of <figref idref="DRAWINGS">FIGS. 1A-1C</figref> is formed from two pieces of metal <b>101</b>-<b>1</b> and <b>101</b>-<b>2</b>. Metal <b>101</b>-<b>2</b> is electrically connected to metal <b>101</b>-<b>1</b> by soldering or attachment means (including mechanical attachment), for example. The patches <b>201</b> are shown as being mounted directly to the vertical walls <b>105</b> of cavity <b>100</b> so that the sides of the patches <b>201</b> facing the walls <b>105</b> of the cavity are electrically coupled thereto. In this embodiment there is no need for the dielectric substrate <b>104</b> to support patches <b>201</b>, but the dielectric substrate <b>104</b> may be utilized if desired. This embodiment typically has a plurality of patches <b>201</b> preferably arranged in two columns with a number of rows as depicted in <figref idref="DRAWINGS">FIG. 2</figref>.

0036<figref idref="DRAWINGS">FIG. 5</figref> shows the width of the slot <b>105</b> and the spacing g of the patches <b>201</b>. There is no relationship between the width of the slot <b>105</b> and the spacing g of the patches <b>201</b>. In the test prototype of <figref idref="DRAWINGS">FIG. 9</figref>, the slot <b>105</b> width is 0.170″, while the gap g between patches is 0.400″.

0037The patches <b>201</b> can be located any distance away from surface <b>101</b>-<b>2</b>. But, ideally, the patches <b>201</b> are disposed very close to the plane of slot <b>102</b> because that enables the cavity depth to be a kept to a minimum.

0038In another embodiment, illustrated by <figref idref="DRAWINGS">FIG. 8</figref>, the AMC (<b>103</b> in <figref idref="DRAWINGS">FIG. 1B</figref>) can be formed with a single column of patches (numbered <b>801</b> in this embodiment) centered on the elongate axis of the cavity <b>100</b>, and the reactive elements (numbered <b>802</b> in this embodiment) are electrically connected between the patches <b>801</b> and each side of the vertical walls <b>105</b> of cavity <b>100</b>. The disadvantage of this configuration is that it requires twice as many reactive elements <b>802</b> compared to the embodiment of <figref idref="DRAWINGS">FIG. 4</figref>. In this figure, metal <b>101</b>-<b>2</b> is omitted for clarity's sake, but the slot <b>102</b> therein is represented by the two dashed lines.

0039Turning again to <figref idref="DRAWINGS">FIG. 6</figref>, when the AMC <b>203</b> is loaded with a plurality of variable varactors <b>601</b> (each reactive element <b>202</b> of <figref idref="DRAWINGS">FIG. 2</figref> in the embodiment of <figref idref="DRAWINGS">FIG. 6</figref> is embodied as a variable varactor <b>601</b> in this embodiment, <figref idref="DRAWINGS">FIG. 6</figref> showing a cross section view through one of the pairs of patches <b>201</b>), it adds a capacitance to the grid of patches, and it has the same effect on the CBSA performance as the capacitor <b>501</b> of <figref idref="DRAWINGS">FIG. 5</figref>, but the varactor <b>601</b> it is tunable by an applied voltage <b>602</b>. The adjustable variable reactor <b>601</b> allows the CBSA frequency of maximum antenna gain and efficiency to be tuned over a wide frequency band. The voltage <b>602</b> can be applied by running a wire to the varactor through an opening <b>603</b> in the bottom of the CBSA cavity <b>100</b>. A simple control scheme is shown in <figref idref="DRAWINGS">FIG. 6</figref>, where two varactors <b>601</b> are connected cathode to cathode to provide the variable capacitance. Such cathode to cathode varactors are available in a three-lead package from most varactor manufacturers (e.g. Skyworks).

0040When the AMC is loaded with NFCs <b>701</b> (as shown in <figref idref="DRAWINGS">FIG. 7</figref>), there are a number of control lines or wires <b>702</b> that supply voltages to the NFC <b>701</b>. Those lines or wires <b>702</b> are connected preferably to a multi-source voltage supply <b>703</b> through an opening <b>704</b> in the bottom of the CBSA cavity <b>100</b>. The preferred negative inductance varies with the cavity dimensions, the AMC dimensions and the preferred operation frequency. For operation at a given frequency f, the AMC's equivalent circuit parameters of inductance L and capacitance C satisfy the equation for a parallel LC circuit resonance, f=1/(2π√{square root over (LC)}) where the capacitance is due to a combination of the edge-to-edge capacitance between the metal patches and the parallel-plate capacitance between the patches and the AMC's ground plane <b>101</b>. The inductance is the parallel combination of the substrate inductance and the load inductance. The substrate inductance is approximately L_sub=8.8d nH*d where d is the AMC thickness in cm. Then the negative inductance is limited to be less than the negative of L_sub, i.e. LNFC&lt;−L_sub. So a preferred range is −30 nH*d(cm)&lt;LNFc&lt;−8.8 nH*d(cm)

0041The NFC <b>701</b> has been implemented in the test article of <figref idref="DRAWINGS">FIG. 9</figref> as a Negative Impedance Inverter having a negative inductance between −70 and −45 nanohenrys and a negative resistance of −7 to −4 ohms as described in “Wideband Artificial Magnetic Conductors Loaded With Non-Foster Negative Inductors” by Gregoire et al. IEEE Antennas and Wireless Propagation Letters Vol. 10, 2011, which is incorporated by reference herein.

0042A schematic diagram of the preferred embodiment of the NFC <b>701</b> is shown by <figref idref="DRAWINGS">FIG. 11A</figref> and that NFC is described in greater detail in U.S. patent application Ser. No. 13/441,730 filed Apr. 6, 2012 and entitled “Differential Negative Impedance Converters and Inverters with Tunable Conversion Ratios”, the disclosure of which is hereby incorporated by reference.

0043<figref idref="DRAWINGS">FIGS. 9A-9E</figref> are photos of an AAMC-CBSA test article. In this embodiment the cavity is 39 inches long (dimension <b>110</b> in <figref idref="DRAWINGS">FIG. 1B</figref>), by 3 inches wide (dimension <b>108</b> in <figref idref="DRAWINGS">FIG. 1C</figref>) and 1.5 inches deep (dimension <b>109</b> in <figref idref="DRAWINGS">FIG. 1C</figref>). The size of the slot <b>102</b> is 39 inches long by 0.170 inches wide. Dielectric <b>105</b> (see element <b>104</b> in <figref idref="DRAWINGS">FIG. 1B</figref>) preferably is 1 inch thick Rohacell structural foam so the grid of patches of the AMC <b>103</b> is located 0.5 inch below the plane of the slot. The ground plane <b>101</b> (metal <b>101</b>-<b>1</b> and <b>101</b>-<b>2</b> together) is 48 inches by 36 inches in this test article. This embodiment uses NFCs <b>701</b> as shown in <figref idref="DRAWINGS">FIG. 11A</figref> for the AMC.

0044<figref idref="DRAWINGS">FIGS. 10A and 10B</figref> are plots of the measured input reflection coefficient magnitude of the AAMC-CBSA test article of <figref idref="DRAWINGS">FIGS. 9A-9E</figref>. <figref idref="DRAWINGS">FIG. 10A</figref> compares the unloaded cavity to the NFC AAMC at 2.0-volt bias. <figref idref="DRAWINGS">FIG. 10B</figref> shows the response of the NFC AAMC with different biases from 1.5 to 2.2 volts. This data shows the input match of the CBSA in the VHF/UHF band is significantly improved in the VHF/UHF band with the NFC AAMC loading, and it can be tuned over a wide range.

0045<figref idref="DRAWINGS">FIG. 11A</figref> is a schematic diagram of preferred embodiment of a NFC, which NFC is described in detail in U.S. patent application Ser. No. 13/441,730 filed Apr. 6, 2012 and entitled “Differential Negative Impedance Converters and Inverters with Tunable Conversion Ratios”, the disclosure of which is hereby incorporated by reference. <figref idref="DRAWINGS">FIGS. 11B and 11C</figref> depict measured circuit values for the NFC <b>701</b> used in the AAMC-CBSA test article of <figref idref="DRAWINGS">FIGS. 10A-10E</figref>.

0046This concludes the description of a number of embodiment of the present invention. The foregoing description of these embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.